Nicola Sturgeon has hardly made a secret of her belief that Scotland's different attitude to the EU possibly justifies a second independence referendum.

At Westminster and Holyrood in recent months there's been a building sense that she has made up her mind to call a vote.

Those close to her are careful to emphasise that there is no final decision. She, herself, is at pains to say she has not reached a conclusion. But even if it's still an "if", she seems clearer on the "when".

In a documentary tonight, the first minister told me the "common sense time" for such a vote would be in autumn 2018. Others have speculated as much, but she is in charge. And her more explicit remarks display how expectation is building.

If Ms Sturgeon is now willing to discuss the timing of a second vote in public, consideration of another independence referendum is far beyond the hypothetical.

The crucial facet of that calculation is that the SNP believes its best chance of winning is before the EU negotiations are complete.

Senior sources suggest Theresa May will be at her most vulnerable when the UK government is consumed with the Brexit negotiations, and that if Scotland is to be offered independence again, that choice must be made before the UK has actually left the European Union.

There is though not just the problem that just as her supporters may be enthused, many other Scots will be enraged by being asked to go there again.

Just as the first independence referendum galvanised Scottish politics, for others it was divisive, even distressing.

But also, it's up to the Westminster government to permit another referendum.

There are huge risks for them in denying it, but ministers in London certainly would not grant a vote at the time of the SNP's choosing without a fight.

My interview with Nicola Sturgeon will be broadcast on Britain's Biggest Deal on BBC 2 at 21.00 GMT. I have also spoken to Boris Johnson, David Davis and Tony Blair among others.